High speed digital inkjet printing systems have recently found much success in competing with traditional analog printing presses in applications such as commercial printing and publishing. In particular inkjet printing systems which employ water-based pigment inks can rival the print quality and productivity of printing systems which are based on analog impact printing, such as lithography, gravure, and flexographic printing processes. The ability of digital inkjet printing systems to cost-effectively print either short run lengths or continuously variable information offers distinct advantages over analog impact printing processes which require the presses to be idled in between print jobs in order to change over the printing plates or cylinders. More recently, high productivity inkjet printing systems have targeted decor and package printing with the same advantages. However, décor and package printing, unlike commercial printing and publishing, commonly employ water-impermeable plastic substrates in addition to paper-based substrates. Plastic substrates are particularly challenging for water-based inks due to the difficulty in getting the inks to wet and adhere to such substrates, which are typically designed and chosen to repel or otherwise present a barrier to water. Although solvent-based and radiation curable (UV-curable) inkjet inks have been used with some success on plastic substrates, they are limited to certain types of inkjet printing systems, such as piezoelectric drop-on-demand systems, and come with health, safety, and environmental concerns compared to water-based inks.
When water-based inkjet inks useful for high speed digital inkjet printing, especially inks that comprise more than about 80 wt % water and less than about 10-15 wt % of organic co-solvents, are deposited on commonly available plastic substrates used for décor and packaging applications, the ink drops tend to either bead up or flow across the surface of the substrate, resulting in image quality artifacts known as mottle, coalescence, and intercolor bleeding. In addition, water-based inks with levels of co-solvents greater than about 10-15 wt % are very difficult to completely dry, resulting in a sticky or tacky ink layer.
WO 2009/113097 A2 discloses a process for printing on a plastic film with water-containing inks. However, the water-containing inks disclosed in this publication are intended for flexographic or offset printing, and are not suitable for high speed inkjet printing due to their high viscosities. There is no teaching as to how these types of inks can be modified for use in a high speed inkjet deposition system.
Another approach to printing directly onto hydrophobic impermeable substrates with water-based inkjet inks is disclosed by Higgins, et al., in U.S. Pat. No. 8,398,226. This process employs water-based inks comprising certain stimuli-responsive additives to permit direct printing onto untreated impermeable plastic. These inks rely, for example, on a thermal phase-change of an incorporated microgel, which causes the inks to rapidly viscosify when a heated ink is deposited onto a relatively cool substrate. In practice the inks need to be kept at an elevated temperature in a low viscosity state prior to printing, and the substrate needs to be held below the phase-change temperature of the water-based ink containing the microgel additive. This imposes additional and expensive constraints on the overall printing system.
Water-based pigment inks suitable for printing directly onto a vinyl medium for signage or display applications are disclosed in WO 2011/028201 A1. However, these inks contain mixtures of dispersed polymer particles that require the printed substrate to be heated above 50° C. to melt and fuse the inks to the vinyl medium. The temperatures required for the fusing step are problematic in that many water-impermeable plastic substrates useful for packaging and décor applications are prone to wrinkling or melting at temperatures as low as 50° C.
Another method for printing on plastic films with water-based pigment inks is disclosed in U.S. Pat. No. 8,500,264 B2. This method involves the steps of depositing water-based inks onto an untreated plastic film and heating the plastic film surface to 40° C. or greater. The inks further comprise “a silicone surfactant, an acetylene glycol surfactant, a pyrrolidone derivative, and a thermoplastic resin.” However, as the examples in this patent clearly demonstrate, when the inventive inks are deposited on a representative plastic film at ink coverages greater than about 60 to 70%, the “bleeding” between colors or the image non-uniformity of solid areas (“mottling”) is judged as “bad”. This severely limits the applicability of this method for most packaging applications for which much higher ink coverages are desired.
U.S. Publication No. 2014/0160211 A1 discloses a method for printing with water-based ink compositions onto a non-absorptive medium while simultaneously heating the medium. The water-based inks further comprise thermoplastic resin particles and first and second co-solvents with specified boiling points and surface tension properties. With many water-impermeable plastic substrates, especially very thin flexible films used for packaging applications, the application of heat while printing can adversely affect the dimensional stability of the film, leading to poor color-to-color registration.
U.S. Publication No. 2013/0187998 A1 discloses water-based inkjet inks with improved scratch resistance that comprise polymer particles and a hydroxyl amine compound that are intended to be printed directly onto non-absorbent polypropylene substrates, such as a non-woven fabric. A key limitation of this method is that the ink drops are between 60 pL and 120 pL, which is much too large to meet the print quality and resolution requirements for many applications, and the viscosity of the ink is greater than 4 mPa-s, which further limits the rate at which ink drops are deposited.
Yet another approach to printing onto impermeable substrates is disclosed in U.S. Pat. No. 8,076,394. This method utilizes the combination of a reactive fixing fluid and water-based inks comprising a specific co-reactive polymer species. When such inks are deposited on substrates which have been pre-treated with the reactive fixing fluid, the inks are claimed to rapidly increase in viscosity, or gel, thereby immobilizing the ink drop. There are several problems with this approach. First, the fixing fluids useful in this printing system comprise boric acid, borax, or copper sulfate, which are problematic in certain food packaging applications. Second, inks comprising the exemplified co-reactive polymer species are limited to those which comprise specific co-solvent humectants, and which require effective levels of the co-reactive polymer species, the combination of which results in inks with viscosities greater than about 10 mPa-s, making them disadvantaged for high speed inkjet depositions systems.
The use of an adhesion layer between an impermeable substrate and an ink-printable layer is disclosed in WO 2012/102737 A1. However, these layers are formed using melt extrusion or co-extrusion processes, and are intended for printing with either UV-curable or latex inkjet inks. As noted above, UV-curable inks are undesirable for health and safety reasons, and latex inks require a secondary heating and fusing step which is problematic for thin, low-melting substrates.
EP 2692536 A1 also discloses the use of an adhesion-promoting layer for use on polyolefin packaging films in combination with an ink-receptive layer suitable for water-based inks. An undesirable feature of this solution is that it is limited to polyolefin substrates to which an adhesion-promoting layer has been formed by either a co-extrusion or lamination process. Further, the exemplified ink-receptive layers are each relatively thick, e.g., 10 micrometers, which can adversely affect the cost and performance of very thin film structures.
EP 2617577 A1 discloses a solution to improving the adhesion of a water-based inkjet ink when printed onto hydrophobic polyolefin films. One option includes surface treating the polyolefin film to introduce polar hydrophilic groups, for example with a corona or plasma treatment discharge process, followed by depositing a water based inkjet ink onto the surface treated polyolefin film. Also disclosed is a second surface treatment, e.g., corona or plasma discharge, after inkjet recording, followed by the application of a protective over coating which contains a wax. When the inks contain specific co-solvents the protective overcoat is not required, and when the protective overcoat is employed, the first surface treatment is not required. In either embodiment, a separate ink-receptive layer is neither required nor disclosed with this system. However, the present inventors have found that hydrophilic surface treatments, such as corona or plasma discharge are insufficient to control lateral ink spread on impermeable films when water-based inkjet inks are deposited thereon, especially in areas of high ink coverage, as will be illustrated and disclosed in more detail below.
A “fixer fluid” comprising a cationic polymer and a high boiling organic co-solvent for use as a pre-coating for printing onto vinyl substrates with “anionic aqueous inks” to enable printing on non-porous substrates is disclosed in WO 2014/042652 A1. However, as disclosed hereinafter, the use of high-boiling organic co-solvents is undesirable, and ink-receptive fixer layers comprising cationic polymers are not suitable for high speed printing with anionically stabilized water-based pigment inks.
To overcome these problems and limitations of printing onto water-impermeable substrates with water-based inkjet inks, particularly those comprising anionically stabilized pigments, the surface of the substrate is pre-coated with water-based ink-receptive compositions comprising multivalent metal salts and hydrophilic binders. Dannhauser and Campbell, in U.S. Publication No. 2011/0279554 A1, disclose an inkjet receiving medium suitable for high speed inkjet printing which includes a substrate having a topmost layer coated thereon comprising a water soluble salt of a multivalent metal cation and a hydrophilic polymer binder. However, even the step of applying a water-based layer comprising multivalent metal cations, such as those disclosed in U.S. Publication No. 2011/0279554 A1, to an impermeable plastic substrate can still be problematic due to the poor wetting and adhesion characteristics of most untreated plastic surfaces by water-based coating fluids. A further problem that has been encountered is that non-volatile co-solvents known as humectants that are commonly added to inkjet inks cannot absorb into the impermeable substrates, which can lead to a prolonged sticky or tacky feel to the surface of the dried ink layers. Yet another problem that is encountered is insufficient layer-to-layer adhesion or intra-layer cohesion when such printed films are further post-coated or laminated to produce the final multi-layered package or article. Thus there is a need for an improved method of forming multi-layered printed images with water-based inkjet inks on impermeable substrates which addresses the problems and limitation listed above.
In addition to the aforementioned objectives, in view of the overall health, safety, and environmental considerations of the manufacturing processes and intended applications of the present invention, especially for flexible film food packaging and labels, it is an overarching objective of this invention to use water-based tie-layers, water-based ink-receptive layers, and water-based pigment inks, which contain less than 15 wt % of organic co-solvents and which do not contain radiation curable sensitizers, initiators, monomers, or oligomers.